suckage
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suckage
Alright guys i've got a 78 f150 with the 300 inline 6 in it. The motor is getting horror movie gas mileage - somewhere right around or under 10 mpg. I was messing with the air-fuel screw and found that the motor runs almost un-noticeably different no matter how far out i turn it. The only change is when i literally take the screw out which then of course causes a vacuum leak but even then the motor stays running which i would think it should not. With the motor running i climbed up top and shined a flashlight down the barrel. When i blipped the throttle the jet squirted a mist of fuel, however it wasnt sucked around into the intake like expected. some of it was, but some of it simply fell onto the intake. I'm wondering what is preventing the flow. Would the problem be solved simply by taking the intake off and sandblasting it and trying to polish the inside? I had planned on doing that anyway to help increase the flow just a little bit but I wouldn't think that a slightly rusted intake would be the only problem here. Sorry for the long post I'm just trying to get it worked out so i can afford to drive this thing farther than 40 miles away from the gas station.
Post Fiend
Joined: Mar 2011
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From: Lynchburg, VA
Are you driving with a light foot, or a heavy one? Driving with a heavy foot, in town, 10mpg or less would certainly be expected.
That screw you were playing with is the idle mixture screw. It has no effect off idle.
If you're just bliping the throttle, the likelihood of all of the fuel from the accelerator pump instantly atomizing is very low. Your results are to be expected.
The intake needs to be rough. The roughness creates turbulence, which helps the fuel vaporize.
That screw you were playing with is the idle mixture screw. It has no effect off idle.
If you're just bliping the throttle, the likelihood of all of the fuel from the accelerator pump instantly atomizing is very low. Your results are to be expected.
The intake needs to be rough. The roughness creates turbulence, which helps the fuel vaporize.
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I drive it with as light of a foot as possible. I mean if i hit a patch of ice in the parking lot i might break them loose every once in a while but overall i drive with a very light foot. and the 10 mpg is at the most. Not in city either. I checked for leaks, but the tank and lines are all new. And whenever i turned the screw i revved it. No difference there either. Also, when i rev it it hesitates for almost a second before revving up. when it's cold my gas pedal acts as a kill switch it is so bad. Also, i'm not looking for the fuel to be atomized, but It looked like a good portion of the fuel just dropped onto the "pad" in the intake directly below the carb, rather than being pulled towards the motor.
Post Fiend
Joined: Mar 2011
Posts: 8,501
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From: Lynchburg, VA
Like I said, the screw controls idle mixture only, so it won't affect anything other than idle.
Were you seeing a solid stream of fuel from the accelerator pump throughout the throttle travel? Hesitation is often caused by bad accelerator pump diaphragm or check valve. Might be carb rebuild time.
Were you seeing a solid stream of fuel from the accelerator pump throughout the throttle travel? Hesitation is often caused by bad accelerator pump diaphragm or check valve. Might be carb rebuild time.
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I replaced them right after I rebuilt the plug but no I have not. I will do that tomorrow. If they're charcoaled what would you say is the cause? because the ones i pulled out prior to these were nasty
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Post Fiend
Joined: Mar 2011
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From: Lynchburg, VA
Dark porcelain generally indicates rich a/f mixture. If it is running rich, I'd begin by checking your float height.
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I'm taking the truck to grandpa's garage tomorrow to rip it apart. I have some god-awful vibration coming from the rear end i want to check out, and so then i will pull the plugs and probably end up adjusting the float as well. Will post results when i get them.
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Checked mileage - 16.5 driving through town. Part of my daily trip is a 3 mile stretch around noon which is run while the engine is cold and sucks more gas. Pretty happy but I'll be doing some tweaking and adding some....slight ( 
) mods.
) mods.
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